Was he born overweight? There isn't really a away to keep overweight sims permanently slim. They have to work out on a regular basis to keep the weight off.
Also, if a sim gains weight by eating (having full meals when they have the "Stuffed" moodlet), the game seems to consider their weight gain permanent. Did that happen to him?
And this isn't a question about Supernatural, so I'm moving it to Misc. Help.